Photosynthesis and Plant Physiology in Detail

Green plants make their own food by photosynthesis, and this remarkable process takes place inside tiny green structures within the cell called chloroplasts. Chloroplasts are found mainly in the cells of leaves and other green parts of a plant. They are green because they contain a pigment called chlorophyll, which absorbs sunlight — the energy source for photosynthesis. A chloroplast is enclosed by a double membrane (an outer and an inner membrane) that separates its contents from the rest of the cell, much like the wall of a tiny factory.

Inside the chloroplast are two main regions where the work happens. Floating in the chloroplast is a thick fluid called the stroma, and within the stroma is a system of flattened, disc-like sacs called thylakoids. These thylakoids contain the chlorophyll and are the actual sites where light is captured. By dividing the chloroplast into these compartments, nature keeps the different stages of photosynthesis neatly organised, so each can take place in the right place.

The thylakoids are often stacked one above another like a pile of coins, and each such stack is called a granum (plural: grana). The grana are connected to one another, forming an extensive network of membranes packed with chlorophyll. This arrangement gives a very large surface area for absorbing light, allowing the chloroplast to trap as much sunlight as possible. The more light the chlorophyll in the grana can capture, the more energy is available for making food.

The chloroplast neatly separates the two stages of photosynthesis. The first stage, the light reactions, takes place in the thylakoid membranes, where chlorophyll captures sunlight and converts its energy into a usable form. The second stage, the dark reactions (the Calvin cycle), takes place in the stroma, where that energy is used to build sugar from carbon dioxide. Thus the chloroplast is like a well-organised food factory: the grana are the solar panels, and the stroma is the assembly hall — together they turn light, water, and carbon dioxide into food.

Photosynthesis happens in two stages, and the first stage is the light reactions, so named because they need light to occur. The light reactions take place in the thylakoid membranes of the chloroplast, where the chlorophyll is found. Here, chlorophyll absorbs sunlight and captures its energy. This light energy is the driving force for the whole of photosynthesis, because it is converted into chemical energy that the plant can use to build food in the next stage.

A key event in the light reactions is the splitting of water, called the photolysis of water. Using the captured light energy, water molecules (H₂O) absorbed by the plant are broken apart into hydrogen and oxygen. ("Photo" means light and "lysis" means splitting, so photolysis literally means "splitting by light.") This splitting of water is what releases the hydrogen needed later to make sugar, and it is unique to the light reactions.

One product of splitting water is oxygen gas, which is given off, or evolved. This oxygen evolution is hugely important: the oxygen released by plants during the light reactions is the source of almost all the oxygen in our atmosphere, the very oxygen that humans and animals breathe. So every time a green plant carries out its light reactions in sunlight, it is quietly replenishing the oxygen that supports life on Earth.

The light reactions also store the captured energy in two special energy-carrying molecules: ATP and NADPH. ATP carries chemical energy, while NADPH carries both energy and the hydrogen obtained from water. Together, ATP and NADPH act like charged batteries that hold energy and hydrogen ready for use. They are then passed on to the second stage, the dark reactions, where they power the building of sugar. In short, the light reactions capture sunlight, split water to release oxygen, and produce ATP and NADPH to fuel the making of food.

The second stage of photosynthesis is the set of dark reactions, also known as the Calvin cycle. They are called "dark" reactions not because they happen at night, but because they do not need light directly — they can take place whether or not light is present, as long as the products of the light reactions are available. The dark reactions take place in the stroma, the fluid part of the chloroplast surrounding the thylakoids. Here the energy captured earlier is finally used to make food.

The purpose of the dark reactions is carbon dioxide fixation — turning the carbon dioxide gas from the air into a part of a solid food molecule. The plant takes in carbon dioxide (CO₂) through tiny pores in its leaves, and in the stroma this CO₂ is captured and joined onto an existing molecule. By "fixing" the carbon from the gas into a larger molecule, the plant changes a simple gas into the building block of sugar. This is the key step that brings carbon from the air into living matter.

The carbon dioxide fixation is carried out with the help of a special enzyme called RuBisCO. An enzyme is a substance that speeds up a chemical reaction, and RuBisCO is the enzyme that grabs carbon dioxide and attaches it to a five-carbon molecule to start the cycle. RuBisCO is one of the most abundant and important enzymes on Earth, because almost all carbon that enters the living world passes through this single reaction. Without RuBisCO, plants could not fix carbon dioxide, and the food chain would have no starting point.

After carbon dioxide is fixed, the energy and hydrogen brought in by ATP and NADPH (made in the light reactions) are used in a series of steps to build glucose, a simple sugar that is the plant's food. Because these steps repeat in a circle, regenerating the molecule that captures the next CO₂, the process is called the Calvin cycle. The glucose made can be used by the plant for energy or stored as starch. In summary, the dark reactions fix carbon dioxide using RuBisCO and use the ATP and NADPH from the light reactions to synthesise glucose — completing the making of food.

The rate at which a plant carries out photosynthesis — that is, how fast it makes food — is not fixed; it depends on conditions in the plant's surroundings. Four main factors affect photosynthesis: light intensity, carbon dioxide concentration, temperature, and water. When these factors are favourable, photosynthesis is fast; when any one of them is in short supply, photosynthesis slows down. Understanding these factors helps farmers and gardeners give plants the best conditions to grow well.

Light intensity is one of the most important factors, because light provides the energy for the light reactions. As light intensity increases, the rate of photosynthesis increases too — up to a point. Beyond that point, the rate levels off, because the plant cannot use light any faster than its other processes allow. In dim light, photosynthesis is slow; in bright light (but not so strong as to damage the plant), it speeds up. This is why plants grow better in well-lit places than in deep shade.

Carbon dioxide concentration also strongly affects photosynthesis, since CO₂ is the raw material fixed in the dark reactions. The air normally contains only a small amount of carbon dioxide, so increasing the CO₂ supply usually increases the rate of photosynthesis, again up to a limit. This is why, in greenhouses, growers sometimes raise the carbon dioxide level to make plants grow faster. Temperature matters because the reactions of photosynthesis are controlled by enzymes, which work best at a moderate, warm temperature; if it is too cold the reactions slow down, and if it is too hot the enzymes stop working properly.

Water is needed as a raw material — it is split in the light reactions — and also keeps the plant's cells firm and its pores open. A shortage of water slows photosynthesis, partly because the leaf pores close to save water, cutting off the carbon dioxide supply. At any given time, the factor that is in shortest supply controls the overall rate; this is called the limiting factor. For example, on a bright, warm day, a low carbon dioxide level may be what holds photosynthesis back. By improving the limiting factor, the rate of photosynthesis can be increased.

Plants absorb a great deal of water from the soil, but they use only a small part of it; most of the rest is lost to the air as water vapour. This loss of water in the form of vapour from the aerial (above-ground) parts of a plant is called transpiration. Most transpiration takes place from the leaves, through tiny pores on their surface, though some also occurs through the stem and other parts. Transpiration is a continuous process that goes on as long as the plant has water and the pores are open.

The pores through which transpiration mainly occurs are called stomata (singular: stoma), found mostly on the underside of leaves. Each stoma is a tiny opening guarded by two specially shaped guard cells. The stomatal mechanism is the way these guard cells open and close the stoma: when the guard cells take in water and swell, they curve apart and the stoma opens, letting water vapour out and carbon dioxide in; when the guard cells lose water and become limp, they close the stoma, reducing water loss. In this way the plant can control how much it transpires.

Several factors affect transpiration. Bright light causes the stomata to open, so transpiration increases during the day. A higher temperature speeds up evaporation, increasing transpiration, while high humidity (lots of moisture already in the air) slows it down. Wind carries away the water vapour from around the leaf, so a breeze increases transpiration. By responding to these factors, the rate of transpiration changes through the day and with the weather.

Transpiration has important benefits (significance) for the plant. It helps cool the plant on hot days, much as sweating cools us. More importantly, it creates a suction or pull that helps draw water and dissolved minerals upward from the roots to the leaves — this is the transpiration pull. It also helps minerals move through the plant. However, transpiration has a disadvantage: it causes a large loss of water, and if the plant loses water faster than its roots can absorb it, the plant may wilt or, in severe cases, dry out. So while transpiration is useful, plants must also guard against losing too much water.

A plant needs to move materials from one part to another: water and minerals absorbed by the roots must reach the leaves, and the food made in the leaves must reach all the other parts. To do this, plants have a transport system made of two kinds of tube-like tissues running through the roots, stem, and leaves: the xylem and the phloem. Together these form the plant's "plumbing," carrying substances over long distances so that every part gets what it needs.

The xylem carries water and dissolved minerals upward, from the roots to the stem and leaves. Water enters the roots from the soil and travels up the hollow xylem tubes in one direction — upward. The upward movement is explained by the cohesion-tension theory: as water evaporates from the leaves during transpiration, it creates a tension (pull) at the top; because water molecules stick to one another (cohesion), they form a continuous column, so pulling the top of the column drags the whole column up. In this way, even tall trees lift water many metres against gravity.

The phloem carries food (mainly sugar) made in the leaves to the rest of the plant — to growing tips, roots, fruits, and storage organs. This movement of food is called translocation, and unlike the one-way flow in xylem, it can travel in different directions, depending on where the food is needed. Sugar is loaded into the phloem in the leaves (the source) and unloaded where it is used or stored (the sink), so the direction of flow can change with the plant's needs.

The movement of sugar in the phloem is explained by the pressure-flow (or mass-flow) idea. When sugar is loaded into the phloem at the leaves, water follows it in, raising the pressure there; this high pressure pushes the sugary solution along the phloem tubes toward regions of lower pressure, where the sugar is removed for use or storage. So water flows up through the xylem by being pulled (transpiration pull and cohesion-tension), while food flows through the phloem by being pushed (pressure flow). Together, xylem and phloem keep the whole plant supplied with water, minerals, and food.

Just as animals have hormones that control their growth and activities, plants have natural chemical substances that control how they grow and respond to their surroundings. These substances are called plant growth regulators (also called plant hormones or phytohormones). They are made in very small amounts in one part of the plant and often act in another part, controlling processes such as the growth of stems and roots, the ripening of fruit, the dropping of leaves, and the opening and closing of stomata. Some regulators promote growth, while others inhibit (slow or stop) it.

Among the growth-promoting regulators, auxins are very important. Auxins cause cells to elongate (lengthen), helping stems grow longer, and they control the bending of a plant toward light (so a plant on a windowsill grows toward the sunlight). Gibberellins are another promoting group; they cause great elongation of stems and help seeds to germinate and buds to sprout. A plant treated with extra gibberellins may grow unusually tall, and dwarf plants can be made to grow normally with them.

A third promoting regulator is the group called cytokinins, which mainly promote cell division — the making of new cells. By encouraging cells to divide, cytokinins help in the growth of new shoots and in keeping plant parts fresh and active; they also delay the ageing of leaves. Working together, auxins (cell elongation), gibberellins (stem elongation and germination), and cytokinins (cell division) drive the healthy growth and development of the plant.

Not all regulators promote growth. Abscisic acid (ABA) is a growth inhibitor: it slows down growth, helps the plant rest in unfavourable conditions (such as winter or drought), and causes the stomata to close to reduce water loss; it is sometimes called the "stress hormone." Ethylene is unusual because it is a gas; its best-known effect is to promote the ripening of fruits (which is why one ripe fruit can hasten the ripening of others nearby), and it also causes leaves and fruits to fall. Thus, by balancing promoters like auxins, gibberellins, and cytokinins against inhibitors like abscisic acid, and with the help of ethylene, plant growth regulators finely control the life of the plant.